The problem is that if a protocol has visible parts, middleboxes will try to do stuff with those and not handle them correctly in corner cases, or when the protocol changes (e.g. TLS had issues with middleboxes attempting to verify details of the handshake and breaking or downgrading a connection when it saw options it didn't know, e.g. because a newer TLS version was used. MPTCP extended TCP and had lots of problems with things that expected specific behavior of TCP flags and parameters, where MPTCP would have liked to use them differently. With HTTP there have been issues with proxies not understanding new headers, or things like websockets. ...)

QUIC tries to prevent this by making as little as possible visible outside the encryption: it should look to a middlebox as much as possible like an opaque data stream, and not reveal any details about what's going on inside. Now there is a proposal to add something that is explicitly visible to the network, and people are worried that will come back to bite them in some way if they make an exception now.